This invention relates to apparatus for achieving cryogenic temperatures in cooled devices and, more particularly, to a thermomechanical cryocooler system with a redesigned cold tip arrangement.
A cryocooler is a device that achieves very low, cryogenic temperatures by providing a heat sink whose operation is based upon one of the several thermodynamic heat removal cycles. The cryocooler is connected to a heat load through a cold tip, forming a cryocooler system. The heat load can be any structure that is to be cooled to the cryogenic temperature. One example is a sensor or an electronic device that must be cooled to low temperature to operate properly. In operation of the cryocooler system, heat is conducted from the sensor or electronic device, through the cold tip, and to the cryocooler heat sink.
Cryocoolers have the important advantage that they do not require a reservoir of cryogenic liquid. They can therefore be used after a long period of inactivity or storage, as where a sensor is mounted in a missile that is stored for an extended period and must be capable of being cooled to an operating temperature within a short period of time, typically measured on the order of minutes or less. Cryocoolers are also used where the structure to be cooled cannot be readily provided with a supply of cryogenic liquid, as in a spacecraft.
Heat flow is somewhat analogous to electrical flow, in that an impedance to heat flow can act as an insulator or resistor that reduces the flow of heat. Any impedance to the flow of heat from the cooled structure, through the conductive cold tip and to the heat sink, results in reduced cooling efficiency. To overcome the reduced efficiency, the cryocooler must be made larger and must consume more power. The increased size and power use are undesirable in most applications. Additionally, the thermal impedance lengthens the time required to reduce the temperature of the cooled structure to a preselected value, a major disadvantage where cooldown must be rapidly accomplished as in a missile seeker system.
Cryocooler and cold tip systems have been carefully designed to minimize thermal impedance. The cold tip is made of copper, which has a high thermal conductivity. The cold tip is typically brazed to the plug that closes the end of the cryocooler, to provide a continuous heat flow path. The geometric design of the cold tip and the connection of the cold tip to the cryocooler have been optimized for minimal thermal impedance.
Nevertheless, there is always a desire and need to improve system performance by reducing thermal impedance between the structure to be cooled and the cryocooler. The present invention fulfills this need, and further provides related advantages.